The present application relates generally to a system and method for measuring drawbar force. The present application relates more specifically to the determining or calculating of the magnitude and direction of forces on the drawbar for use with an electronic draft control system.
One of the most common uses of agricultural tractors is to move implements through agricultural fields to cultivate and condition the soil. Implements are commonly connected for towing by the tractor using a three-point hitch or a drawbar. A three-point hitch typically has two bottom lift arms, to which the implement is connected in a rotary manner to selectively pivot about a given hinge axis, and a top link interposed between the tractor frame and the implement to control the angular position of the implement about the hinge axis. The lift arms are moved by actuating a cylinder (or cylinders) interposed between the tractor frame and the lift arms to raise and lower the implement with respect to the ground. Similarly, many trailed implements towed using a drawbar include one or more wheels pivotably connected to the implement in a manner to raise and lower the implement with respect to the ground. Movement of the lift arms and/or implement-attached wheels can be used to establish implement position with respect to the ground.
In today's farming practices, control of the quality of cultivation performed by various ground-engaging devices attached to the frame of the implement can be desirable and sometimes critical. As the implement frame is lowered closer to the ground, the ground-engaging devices or tools dig or cut deeper into the soil and the draft load on the tractor increases. As the frame is raised higher above the ground, the ground-engaging devices dig more shallowly into the soil and the draft load on the tractor decreases. Some implements must remain engaged with the ground during operation such that the tools do not become disengaged from the ground, which is usually implemented by specifying a minimum ground engagement depth. Typically, the operator has a manually operable device in the cab of the tractor that is manipulated to raise and lower the implement accordingly, whether by the three-point hitch, the implement wheels, or a combination of the two. When the operator finishes manipulating the device, the implement remains in the position set by the operator, but will not, however, maintain a desired depth of engagement or implement draft load as the tractor and implement move across the ground. Changes in field contour or soil hardness cause the depth of engagement and/or the draft load to change.
To maintain the implement in a position to achieve a consistent draft load or depth of engagement, the operator must periodically look rearward and observe the implement. If the implement has drifted away from the desired depth of engagement, the operator must manipulate the depth control device to reposition the implement until the desired depth of engagement is reestablished. Similarly, changes that increase the draft load may cause the engine to be bogged down, requiring operator adjustment of implement position to avoid stalling the engine. Therefore, for systems in which the operator can adjust the implement position, periodic or semi-constant (under some field conditions) visual monitoring of the implement position and adjustment of the hitch height is necessary to maintain the desired draft load on the tractor.
The concept of electronic draft control (EDC) systems has been applied to alleviate the need for manual operator hitch adjustments by controlling the position of the connected implement in response to the draft loads applied to the tractor by the implement. EDC systems permit the depth of engagement to be adjusted so that a constant draft load is applied to the tractor to smooth tractor operation. Such control systems typically rely on one or more measuring devices to sense the draft load applied to the hitch by the implement and the implement position, and then adjust the implement position in response. Numerous methods have been used to monitor the draft load on a tractor including: direct measure using a special draft pin fitted in the drawbar, i.e., a load bearing pin that can electrically measure the strain on the pin from a given load; monitoring the hydraulic pressure in hydraulically cushioned drawbars; monitoring the cushion deflection in spring or elastic material cushioned drawbars; and derivation of the draft load from engine and/or wheel torques.
All of the previously mentioned methods for monitoring draft load can be effective so long as the connected implement is essentially coaxial with the tractor since the previous methods measure only the longitudinal draft load force, i.e., the previous methods measure the draft load force along the “common axis.” However, when using a drawbar connection, the tractor and connected implement may not be coaxial, such as when travelling along a curved path, and lateral draft forces can be introduced, i.e., draft forces that are perpendicular to the longitudinal draft forces. The presence of lateral draft forces can lower the longitudinal draft forces typically sensed by the EDC systems and, as a result, the EDC will make control decisions, e.g., raising or lowering of the implement, based on an inaccurate measurement of the draft load, since the EDC system is not provided with, nor factoring in, the lateral component of the draft load. Stated differently, the sensors used in a drawbar hitch EDC system measure force along a specific axis/direction. Only the magnitude of the force along the specific axis/direction is measured and provided to the EDC system. If a force is acting at an angle to the specified measurement axis/direction of the sensor, this force is not measured and only the force component acting along the specified axis is measured.
Therefore, what is needed is a system and method to determine both the magnitude and direction of the draft forces on a tractor with a drawbar connected implement.